The following terms are defined for use in the present disclosure:                Screw—meant here as the type known as a machine screw, a type of fastener featuring a male thread of some named standard which is used for mechanical assembly by matching with a suitable threaded hole        Threaded hole—a fastening feature featuring a female thread formed in a hole or bore of some named standard to accommodate a machine screw        A part—any object, fixture or assembly that is designed to be constructed with screws        Threaded insert—an object featuring a threaded hole, which is designed to be installed in a part in order to accommodate a certain type of screw        Through hardening—a process that increases the strength and or hardness of a metal part to its bulk as well as its surface        Case hardening—a process that increases the strength and or hardness of a metal part to some skin depth below its surface        
Screws are commonly used to produce mechanical assemblies. There are several standards defining their properties such as dimensions, tolerance, pitch, thread form, materials etc. Essentially, screws are only intended to be mated with matching threaded holes; i.e., a metric screw designated M6X1.0 and produced according to the ISO 68-1 standard or a similar metric standard should only be mated with a threaded hole designated M6X1.0 according to the same, or a similar standard. Often a threaded hole is produced in a part by installing a threaded insert in it in some manner. The threaded insert may have properties such as high strength or thread locking that are hard to produce in the part itself and it may also be faster or cheaper to install a pre-threaded insert than to produce a thread in the part itself.
It is not uncommon for users to err and to install a screw forcefully into a miss- matched threaded hole. This mistake happens commonly when two similar sized screws are available to users. For example, the metric M6X1.0 screw has a nominal diameter of six millimeters and a pitch of 1.0 mm and the imperial type 1/4-20 UNC screw has a nominal diameter of 6.35 mm and a pitch of 1.27 mm. When an M6X1.0 screw is turned into a miss-matched 1/4-20 UNC threaded hole, it can take nearly an entire turn for the miss-matched threads to mesh into resistance. Many users will not realize that the screw is miss-matched at that point and try to force the screw in further. A miss-matched screw assembly can damage both the screw thread and the threaded hole, depending on the respective material strength. It may have implications ranging from a compromised strength of the assembly, through a damaged mounting thread on a valuable instrument such as a camera or an electro-optic measuring device, to a jammed screw that cannot be removed from the threaded hole. Another example of a common pair of metric and imperial screw sizes that are readily miss-matched is the metric M4X0.7 and the imperial #8-32 UNC screws.
Threaded holes produced in manufactured parts can be divided into two categories: those meant for use in the assembly of the product by designated personnel or an automated process; and those meant to remain free in the final product and used by the end user for mounting of the product or any other function. The first kind of threaded holes is less likely to suffer from miss-matching of screws as it is used as part of the assembly process, typically in accordance to instructions and subjected to quality control measures. The second kind of threaded holes is subjected to the risk of miss-matched screws, with varying severity. For some cases it would be advantageous to have a threaded hole which is able to accommodate both types of a commonly miss-matched screw pair safely in order to prevent the damage associated with a miss-matched screw installment.